(1) Field of the Invention:
This invention relates to a particulate polymer having excellent characteristics as a powder, and more specifically to a particulate polymer obtained by emulsion polymerization, composed of primary particles having particle sizes of at least 20 .mu.m and/or secondary particles formed by agglomeration of at least some of said primary particles and being in the form of a compact mass substantially free of voids.
(2) Description of the Prior Art:
A polymer latex obtained by emulsion polymerization generally contains polymer particles, the sizes of which are not greater than 1 .mu.m, as emulsifiercovered micelles in a state dispersed and floating in water. Since the particle sizes are extremely small, it is difficult to subject the polymer latex directly to solid-liquid separation for the recovery of the polymer. As conventional methods for recovering a polymer from such a latex, it has been widely practiced (1) to separate the polymer directly as particles by using a spray dryer or (2) to mix a salt or acid in the polymer latex to coagulate same and after heating and solidifying the coagulated mixture, to dehydrate and dry the resulting polymer to recover it as particles.
In order to control the sizes of polymer particles to be obtained, it has also been proposed (3) to solidify a polymer latex after dispersing it, as spherical droplets in a specific solvent (Japanese Patent Laid-Open No. 68385/1977), (4) to form a polymer into particles by adding an organic liquid, which has low solubility in water and cannot dissolve the polymer but can wet the polymer, to a slurry obtained as a result of coagulation of a polymer latex (Japanese Patent Publication No. 5610/1984), (5) to mix a polymer latex with an organic liquid, which has low solubility in water and cannot dissolve the polymer but can wet the polymer, in the presence of a coagulant (Japanese Patent Publication No. 5611/1984), and (6) to disperse and semi-coagulate droplets of a polymer latex in a coagulating atmosphere, to solidify the semi-coagulated droplets in a coagulating liquid and then to dehydrate and dry the resultant polymer particles (Japanese Patent Laid-Open No. 95905/1981), etc.
Method (1) which makes use of a spray dryer is however accompanied by the problems that it requires substantial drying energy since a polymer latex . containing a great deal of water is dried as is and a large volume of water has to be evaporated and, moreover, that it is difficult to control the size and bulk specific gravity of the particles.
Method (2) featuring coagulation and solidification involves the problems that when a usual tank- or tower-shaped coagulation and solidification apparatus is employed; the resulting polymer particles have a broad particle size distribution and especially contain fine particles of smaller particle sizes in a high proportion. As a result, they have poor handling easiness and have a high water content and hence require enormous drying energy.
In the method (3), it is necessary to process a large amount of a solvent employed in the sphere-forming treatment. In addition, there is potential inconvenience that problems such as fish eyes could occur upon processing of the resultant polymer, because spherical latex droplets are coagulated externally and their coagulation is hence uneven. Although not as much as method (3), method (4) is accompanied by the drawback that it requires an addition of 60-500 parts by weight of an organic liquid per 100 parts by weight of the polymer or at least 200 parts by weight of an organic liquid per 100 parts by weight of the polymer especially when a high bulk specific gravity is desired. It is also necessary to process a large amount of an organic liquid employed. In method (5), it is also necessary to add an organic liquid in an amount 1-5 times the volume of the polymer. There is hence a need to process a large amount of the organic liquid. It is accompanied by another drawback in that it requires large facilities since the concentration of the polymer is low during its processing and treatment. Method (6) permits easy formation of spherical particles because the mechanism of its processing is the same as the spray drying method. It is however accompanied by such shortcomings that a limitation is imposed on the sizes of particles and a large apparatus is required since the coagulation of particles takes place in a vapor phase.
At present, the automation of powder metering is improving and greater storage and transportation facilities are being employed. There is hence a strong demand for the development of particulate polymers which can be handled easily from the viewpoint of the blocking phenomenon, i.e., the caking of particles in the course of storage and the clogging of transportation lines due to insufficient flowability of particles. In order to avoid contamination of working environments with particulate polymers, it is also necessary to form them while minimizing the number of fine particles. When a particulate polymer is used as a mixture with other particles, the particulate polymer is required to have a suitable particle size so that the composition of the mixture is not changed due to separation of the mixture into its components during storage or transportation. There has also been a strong demand to develop a particulate polymer having a large bulk specific density and uniform and suitable particle size so as to permit direct feeding of the resulting particulate polymer to an injection molding machine or calender rolls and hence to omit the pelletizing process.
The present inventors carried out extensive research with a view toward providing solutions to the above-mentioned problems. As a result, it has been found that a particulate polymer capable of attaining this objective can be obtained by coagulating a polymer latex in the presence of a coagulant and an organic liquid having low solubility in water, thereby leading to the present invention.